Question 3

College Answer

Although the pass rate for this question was reasonably high the examiners commented on a lack of detailed knowledge within most answers for such a core component of our daily practice. Several candidates failed to provide a normal value and only few provided anything other than 5l/min.

There was a general lack of detail, and at times, some confusion about the Frank Starling effect. Most candidates outlined the major determinants of stroke volume, although many were light on the determinants of each or incorporated incorrect facts. Several candidates did not mention HR as a determinant of CO.

Discussion

Normal values, which the college did not explicitely ask for but clearly still expected in the answer:

• 5L/min (for 70kg adult)
• 2.5 to 4 L/min/m² (indexed to body surface area)

The determinants of cardiac output are:

• Heart rate
  • A higher heart rate increases cardiac output as it multiplies by stroke volume
  • An excessively high heart rate decreases cardiac output by decreasing preload
• Stroke volume, which is in turn determined by preload, afterload and contractility
• Preload
  • Increased preload leads to an increase in the stroke volume 
  • Preload is determined by:
    • Intrathoracic pressure
    • Atrial contribution ("atrial kick")
    • Central venous pressure (RA pressure)
    • Mean systemic filling pressure which depends on total venous blood volume and venous vascular compliance
    • Compliance of the ventricle and pericardium
    • Duration of ventricular diastole
    • End-systolic volume of the ventricle
• Afterload
  • Ventricular radius (End-diastolic volume)
  • Ventricular wall thickness
  • Ventricular transmural pressure
    • Intrathoracic pressure
    • Ventricular cavity pressure
    • Ventricular outflow impedance  and aortic input impedance
    • Arterial resistance
      • Vessel radius
      • Blood viscosity
      • Length of the arterial tree
      • Inertia of the blood column
      • Influence of reflected pressure waves
      • Arterial compliance
• Cardiac contractility:
  • Increased contractility improves stroke volume at any given preload or afterload value
  • Affected by:
    • Heart rate (Bowditch effect)
    • Afterload (Anrep effect)
    • Preload (Frank-Starling mechanism)
    • Cellular and extracellular calcium concentrations
    • Temperature

Vincent, Jean-Louis. "Understanding cardiac output." Critical care 12.4 (2008): 174.

Taylor, Henry Longstreet, and Kenneth Tiede. "A comparison of the estimation of the basal cardiac output from a linear formula and the “cardiac index”." The Journal of clinical investigation 31.2 (1952): 209-216.

Tanner, J. M. "The construction of normal standards for cardiac output in man." The Journal of clinical investigation 28.3 (1949): 567-582.

Grollman, Arthur. "PHYSIOLOGICAL VARIATIONS IN THE CARDIAC OUTPUT OF MAN: XI. The Pulse Rate, Blood Pressure, Oxygen Consumption, Arterio-Venous Oxygen Difference, and Cardiac Output of Man During Normal Nocturnal Sleep." American Journal of Physiology-Legacy Content 95.2 (1930): 274-284.

WIGGERS, CARL J. "Determinants of cardiac performance." Circulation 4.4 (1951): 485-495.

Åstrand, Per-Olof, et al. "Cardiac output during submaximal and maximal work." Journal of Applied Physiology 19.2 (1964): 268-274.